1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method using an electrophotographic method. Particularly, the present invention relates to prevention of misregistration of image formation arising from a toner image.
2. Description of the Related Art
Recently, there is a strong demand for image forming apparatuses to downsize in terms of saving more space while increasing image quality and productivity. As a method for downsizing, there is proposed an image forming apparatus that adopts a kind of intermediate additive color mixing, that is, juxtaposing a color mixture by arranging a plurality of exposure devices and a plurality of developing units side by side on one photosensitive member. This image forming apparatus arranges yellow, cyan, magenta, and black toners side by side within, for example, an n x m (n and m are integers) dot matrix which forms one pixel. The image forming apparatus forms an image by mixing of reflected light beams of the respective toners, that is, juxtaposed color mixture which is a kind of intermediate additive color mixing. This method can greatly downsize the apparatus because it can reduce photosensitive members and charging devices which are necessary for respective color stations in a conventional image forming apparatus.
When the image forming apparatus uses a transfer medium with high slipperiness on the paper surface such as coated paper, slipperiness between the image carrier and the transfer medium changes in the image traveling direction depending on the presence/absence of a toner image. The change of the slipperiness generates a velocity difference between the image carrier and the transfer medium. The generation of the velocity difference between the image carrier and the transfer medium to delay the traveling of the transfer medium results in a local shrinkage in the sub scanning direction within the print image. If the rotational velocity of the image carrier varies and decreases with respect to the transfer medium, an image failure such as a local stretch in the sub scanning direction within the print image or an image streak caused by a blur of image exposure on the image carrier may occur.
To prevent an image failure caused by the slipperiness difference, Japanese Patent Laid-Open No. 11-52758 discloses an arrangement which prints image misregistration prevention dots in almost all the image region.
In Japanese Patent Laid-Open No. 2004-151588, a normal image region where a developer image is formed based on print image information is formed. In addition, a dot dispersed image in which dot developer images each having an area of one or more dots are dispersed is formed upstream of the normal image region in the intermediate transfer medium moving direction within the region of a transfer medium P. Of dots formed in the dot dispersed image forming region, dots whose developer image density based on print image information is lower than a predetermined density form a dot developer image at the predetermined density. Dots whose developer image density is higher than the predetermined density form a dot developer image at a density complying with external information.
However, the image forming apparatus using juxtaposed color mixture cannot superpose dots formed by toner on each other to prevent an image failure caused by the velocity difference between the image carrier and the transfer medium in image formation, which is a problem to be solved by the present invention. Even if it is determined that the developer density of a dot based on print image information is low, no dot dispersed image developer can be superposed on the dot, unlike Japanese Patent Laid-Open No. 2004-151588.
When forming a predetermined dot dispersed image, if a dot dispersed image is formed preferentially to a dot which overlaps print image information, the dot pattern portion of the dot dispersed image seems decolorized at the solid portion of the print image. The image loss roughens the image and degrades the image quality.